Heat-activatable adhesives very often replace pressure-sensitive adhesive tapes in operations where the tapes are required to withstand high temperatures or where very high bond strengths are needed.
Heat-activatable adhesives possess such properties because they have no tack at room temperature and become soft and tacky only with an accompanying increase in temperature. For the mediation of adhesion (for the formation of an adhesive layer), heat-activatable adhesive sheets require application of heat and in general—as for all pressure-sensitive or self-adhesive materials—the application of a certain pressure. In general a distinction is made between thermoplastic and thermally reactive heat-activatable adhesives.
Thermoplastic heat-activatable films are based on the concept that these films are hard at room temperature and then, under heat and pressure, soften, flow on the substrate and then become solid again with accompanying cooling. Thereafter the adhesion to the substrate to be bonded is high, as a result of the wetting process of the adhesive.
In the case of the thermally reactive heat-activatable adhesives, the process that operates is the same in analogy to the thermoplastic heat-activatable adhesives. Additionally here, though, in the course of temperature activation, a crosslinking reaction is initiated which results in the thermally reactive heat-activatable adhesives curing, and hence leads to an increased bond strength.
As a result of the properties described above, heat-activatable adhesives are often used for bonds of metals, or of metals with plastics. This also concerns the electronics industry, with the consequence that heat-activatable adhesives are being used increasingly in the form of diecuts (products from stamping operations).
The production of such diecuts in diecutting processes, however, is not entirely trivial, for reasons including the fact that heat-activatable films cannot, like pressure-sensitive adhesives, simply be transfer-laminated to a desired liner, since at room temperature the pressure-sensitive adhesion is absent. This process generally does not function even with highly graduated release liners in conjunction with a hot-laminating operation.
Nevertheless, within the electronics industry, there is a need to produce such diecuts; in particular to the exclusion of disruptive, unwanted paper fibers, or, for sensitive applications, without silicone. Since a very large number of heat-activatable adhesives are typically furnished with glassine release liners, the aforementioned problem is manifested to a particular degree. There is therefore a need for a general diecutting process for heat-activatable adhesives that operates to the exclusion of paper release liners.
It is an object of the invention, therefore, to offer a method of producing diecuts from heat-activatable adhesives that avoids, or at least minimizes, the disadvantages of the prior art. A particular aim is to avoid the presence of paper as a carrier material in the diecutting operation itself. Advantageously it is to be possible to optimize the partability of the diecuts from the carrier material for the particular intended application.